A conventional inkjet printing system includes a printhead, an ink supply which supplies liquid ink to the printhead, and an electronic controller which controls the printhead. The printhead ejects ink drops through a plurality of orifices or nozzles and toward a print medium, such as a sheet of paper. Typically, the orifices are arranged in one or more arrays such that properly sequenced ejection of ink from the orifices causes characters or other images to be printed upon the print medium.
Between drop ejections, ink in the orifices suffers from evaporation. With the evaporation, material especially dye can precipitate out of the ink, which can result in the formation of a viscous plug in the orifice. Raising the ink viscosity can slow evaporation by reducing the diffusion rate of water from the bulk ink. If too much dye, or other material, precipitates out or the viscous plug that forms is too big, poor first-drop out ink drop volumes or weights may happen when ink is ejected from the orifice. If a printhead is left at an excessively high temperature for a period of time when it does not eject ink, the time may be short before the ink thickens and becomes a defect-producing nozzle obstruction.
One method to reduce this thickening of ink or prevent formation of a viscous plug is to eject ink, which may or may not be thickened, out of the nozzles a multitude of times at regularly scheduled intervals, where the ejected ink is not part of printing images onto a media. This process is also referred to as spitting. Generally, spitting occurs either into a spittoon ink collection device or on the margins of the paper. When ink is spit onto the margins, the margins need to be trimmed away from the printed image in a post-printing operation that adds cost and time to printing. Often for ink formulations with poor ink thickening properties no drops may be ejected on the first ten, hundred or even thousand energizing of a resistor, but the nozzles do eventually recover.
Another method to improve ink ejection performance is to alter ink formulations in order to change the characteristics of the ink. However, this can constrain the overall ink formulation and is not always feasible with competing interest, e.g. image gloss, fast drying or adhesion to the media, in ink formulation.
Since, some warming of the printhead, eg. at 35 to 50° C., is normally needed to maintain consistent drop weight during printing, another approach to improve ink ejection performance consists of warming the printhead die to high temperatures, e.g. above 50° C., and maintaining the printhead die at a substantially constant temperature whether ink is being ejected or not. While such an approach can be effective, excessive temperature elevation of the printhead can reduce printhead life by accelerating diffusion of ink into adhesive joints. Further, excessive warming of the printhead adds to the cost to the printer operation.
Therefore, there exists a need to improve ink ejection performance without the disadvantages associated with known approaches.
It is understood that the above-described embodiments are merely illustrative of the possible specific embodiments which may represent principles of the present invention. Other arrangements may readily be devised in accordance with these principles by those skilled in the art without departing from the scope and spirit of the invention.